It is known that, by adding a phenol-based antioxidant to polyolefin such as polyethylene and polypropylene, oxidation deterioration of a molded polyolefin article generated by light or heat is remarkably decreased.
A phenol-based antioxidant is usually a powder having a melting point of around 100° C. and a median diameter of 5 to 70 μm, is added to polyolefin with better flowability through a hopper without being melted, but dust-proof measure was necessary upon addition to polyolefin due to a dust generated from a powder of a phenol-based antioxidant.
Under such the circumstances, a process for preparing a pellet of mixing tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxymethyl]methane which is one kind of a phenol-based antioxidant, and an antacid additive such as stearate, heating the mixture to 115° C. to melt a part of the phenol-based antioxidant, and mixing a 2.5 mm pellet-like phenol-based antioxidant and an antacid additive with an extruder is disclosed in JP-A No. 5-179056 (e.g. Example 1). It is disclosed that the pellet is dust-free even when a binder such as paraffin which is not desirable to polyolefin is not used, and there is no disorder upon addition to polyolefin.
However, polyolefin obtained by melting and kneading a pellet obtained by heating a phenol-based antioxidant to 100° C. or higher in advance has a problem that heat stability is uneven, and it has also a problem that the polyolefin must be extrusion-molded using an expensive extruder.